Fails with queries with more than 32768 arguments

[Fuyukai]

• asyncpg version: 0.10.1
• PostgreSQL version: 9.6.1
• Python version: 3.5.3
• Platform: GNU/Linux
• Do you use pgbouncer?: No
• Did you install asyncpg with pip?: Yes
• If you built asyncpg locally, which version of Cython did you use?: N/A
• Can the issue be reproduced under both asyncio and
  uvloop?: Yes

When sending queries with more than 32768 arguments, the execute fails with this error:

  File "asyncpg\protocol\protocol.pyx", line 162, in bind_execute (asyncpg/protocol/protocol.c:57595)
  File "asyncpg\protocol\prepared_stmt.pyx", line 100, in asyncpg.protocol.protocol.PreparedStatementState._encode_bind_msg (asyncpg/protocol/protocol.c:53533)
ValueError: number of arguments (90000) does not match number of parameters (24464)

Note that 90000 % 32768 is 24464.

EDIT: This is actually caused by overflow on 32768, not 65536.

[1st1]

From the Postgres documentation of its binary protocol, here's a description of the Bind message:

Bind (F)
    ...
    Int16
        The number of parameter format codes that follow (denoted C below). 
        This can be zero to indicate that there are no parameters or that the 
        parameters all use the default format (text); or one, in which case the 
        specified format code is applied to all parameters; or it can equal the 
        actual number of parameters.
    ...

Int16 is a signed 16 bit integer, so 32768 is how many arguments Postgres can accept.

We do need to handle this case and raise a proper exception in asyncpg.